When the natural lens of the human eye becomes impaired through, for example, cataracts or injury, it is common practice to replace the natural lens with an IOL. One way to accomplish this is to form a relatively long incision in the eye and remove the natural lens in one piece. However, presently a much more common way to accomplish this is to form a shorter incision in the eye and insert a probe or a phaco tip of a phacoemulsification instrument through the incision into the eye to break up the natural lens using ultrasonic energy. The lens fragments are then aspirated from the eye through the relatively short phaco incision and the phaco tip is removed.
The dimension of the incision, which is commonly referred to as a phaco incision, in the eye through which the phaco tip is inserted is typically no larger than about 2.8 mm or about 3.2 mm so that the phaco tip will fit somewhat snugly through the incision. It is generally recognized that the larger the lens-removal incision, the greater the trauma to the eye and the longer the patient recovery time. Other complications may also be caused by a large incision such as the need to remove the lens in one piece. About 70 percent of the lens-removal processes currently use phacoemulsification techniques. Of course, with only a small incision being needed for phacoemulsification lens extraction, it is desirable that IOL's be available and used that can be inserted through the small phaco incisions.
A typical IOL includes an optic, usually having a diameter of about 6 mm, and fixation members coupled to (or formed with) the optic to fix the optic within the eye in the region of the extracted lens. IOL's are of two basic types, those having a hard or rigid optic formed, for example, of polymethylmethacrylate (PMMA) and those having a deformable optic which is constructed of a deformable material such as silicone, hydrogel, or an acrylic. If a hard IOL is used, the small phaco incision must be enlarged to approximately the diameter of the hard optic, in order to permit the hard optic to be inserted through the incision and much of the advantage of phacoemulsification lens extraction is thereby obviously lost.
There are two known classes of prior art IOL's which can be deformed (e.g., folded or rolled) to pass through a scleral tunnel incision having a dimension of about 3.2 mm. A scleral tunnel incision is a single incision in the sclera without any additional incision which crosses or intersects the single incision. The first of these IOL's has an acrylic optic with a refractive index of about 1.47 or greater. The acrylic IOL's cover a full diopter range of about 12d to about 24d, but do not have as high an elongation as is desired for IOL insertion through a small phaco incision. Elongation is defined as (L.sub.d /L.sub.u) times 100 where L.sub.d is the maximum change in length from the unstressed condition to the breaking point and L.sub.u is the unstressed length. A high elongation is desired so that the optic can be caused to resiliently stretch and flow to assume a small cross sectional configuration for passage through a small phaco incision. For example, a currently known acrylic optic may have an elongation of only about 150 percent.
The second of these classes of IOL's has a silicone based optic. In one known silicone based optic the refractive index is only about 1,408. Accordingly, it is necessary to have a relatively large maximum cross sectional area of about 5.1 square millimeters in order to provide an IOL of only 12 diopter power. Although higher diopter powers can be constructed with this silicone based material, the higher powers require a correspondingly greater maximum cross sectional area with the result that they will not ordinarily pass through a scleral tunnel incision having a dimension of only about 2.8 or about 3.2 mm in the eye. More specifically, the present inventors understand that it is highly unlikely that this type of silicone IOL is implantable, using a stainless steel folding forceps-type inserter, through a 3.2 mm scleral tunnel incision in powers over 14d and that in powers over 15d implantation through a 3.2 mm scleral tunnel incision into the eye is essentially not possible. Implantation of an IOL of this type having a power of 20d or greater is not possible.
A second kind of known silicone based optic is disclosed in Fedorov et al U.S. Pat. No. 4,647,282. One of the silicone based materials disclosed in this patent is said to have a refractive index of 1.480. However, this silicone based material has a percentage of elongation of only 130 percent, and the patent lacks, among other things, specific geometrical teachings as to how the optic is to be constructed.
A third kind of known silicone based optic is disclosed in U.S. Pat. No. 5,236,970 and entitled OPTICALLY CLEAR REINFORCED SILICONE ELASTOMERS OF HIGH OPTICAL REFRACTIVE INDEX AND IMPROVED MECHANICAL PROPERTIES FOR USE IN INTRAOCULAR LENSES. At present, a known 16 diopter power optic of this material has a maximum cross sectional area of about 5.3 sq. mm.
Another important consideration in producing small deformable IOL's is the placement of the fixation members or haptics, used to assist in retaining the optic in the eye, relative to the optic. Because of strength, molding and other material concerns, it has heretofore been considered necessary to secure the proximal end of the fixation members in the optical zone of the optic of such IOL's, for example, when the IOL is constructed so as to be deformed and placed in the eye through a small incision. This construction has at least one disadvantage, a portion of each of the fixation members exists in the optical field defined by the optic and, therefore, can interfere with the patient's vision. This problem is exacerbated by the current trend to smaller and smaller IOL optics. It would be advantageous to provide an IOL including one or more fixation members, for example, of the filament type, in which the optical zone of the optic is completely free of the fixation members.